1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device and more particularly to a heat treatment for a semiconductor device having an oxide film incorporated with impurities of boron and phosphorus.
2. Description of the Prior Art
A semiconductor device of the type manufactured in recent years is provided with a fine circuit pattern and has a stage depth made deeper after an etching process. In particular a dynamic random access memory device has a stacked storage node and a circuit formed by a multi-layer circuit method. Therefore, it is necessary to make the stage depth flat for an easy formation of a lithography pattern or an easy execution of the etching process on the top layer. The top layer can be made flat by the step of depositing of an oxide film containing boron and phosphorus, that is, borophosphosilicate glass (referred to BPSG hereinafter) on the top layer and heating at a temperature higher than 900.degree. C. to subject the BPSG to a reflow process. This method is disclosed in, for example, Oki Denki Research Development 130, Vol. 53, p. 79 (April, 1986) and 140, Vol. 55, p. 123.
A conventional heat treatment has been carried out in a following way: FIG. 15(A), 15(B), and 15(C) are schematic views showing a manufacturing chart of the conventional heat treatment for the oxide film and illustrating the positions of substrates put in a heating boat against the furnace tube during the various heat treatments. With reference to FIG. 15, a reference numeral 81 denotes a furnace tube and a reference numeral 82 denotes a heating boat. A reference numeral 83 denotes a substrate having the BSPG deposited thereon on the heating boat 82. FIG. 16 is a graph showing a heat profile according to the conventional heat treatment of the oxide film.
With reference to FIG. 15, the conventional heat treatment of the oxide film deposited on the substrate is described in connection with the various manufacturing steps. At a boat putting-in process, the substrates 83 are arranged on the heating boat 82 as shown in FIG. 15 (A) and are put in the furnace tube 81 as shown in FIG. 15 (B). After the heating up process 92, the substrates are subjected to the heat treatment at a temperature higher than 900.degree. C. during a heat treatment process 93 as shown in FIG. 16. The BPSG deposited on the top layer shows a lower viscosity coefficient and becomes flat with time due to the surface tension. Finally, the substrates 83 on the heating boat 82 are pulled out from the furnace tube 81 to the outside air during the boat putting-out process as shown in FIG. 15 (C).
In the conventional reflow method mentioned above, grains having a diameter of 0.1 .mu.m to 10 .mu.m are generated on the surface of BPSG by a chemical reaction of the boron and/or phosphorus in the BPSG with oxygen and/or vapor in the air or by a crystal growth in the BPSG. The observation of the precipitated grains by a reflection high energy electron diffraction analyzer or by an electron probe X-ray micro analyzer (EPMA) indicates clearly that the precipitated grains are composed of the crystal grains including a large amount of phosphorus and oxygen. The present inventors reported this finding at the lecture No. 26a-D-5 of fall meeting of the Japanese Applied Physics Soc. No. 51, 1990.
FIG. 17 shows mechanisms for the generation of grains. FIG. 17 (A) shows a mechanism of the grain growth in the gas phase and FIG. 17 (B) shows a mechanism of the grain growth in the solid phase. In FIG. 17, a reference numeral 101 denotes the BPSG and the reference numeral 102 denotes the precipitate grains. With reference to FIG. 17, the mechanism of the crystallization of grains will be described. As shown in FIG. 17 (A), oxygen and water vapor in air react with boron and phosphorus evaporated from the BPSG 101 and promote the grain growth of the precipitated grains 102 at the gas phase as shown in FIG. 17 (B). The BPSG at a critical temperature range causes the phosphorus oxide and the boron oxide to react with each other at the solid phase and to generate the precipitated grains.
The substrate according to the prior art causes the precipitated grains to prevent the surface of the BPSG from being flat. As a result, the conventional manufacturing method for a semiconductor device generates pattern defects or etching defects resulting from the precipitated grains and therefore has a very low manufacturing yield.